Investigators involved in modern electronic and electromechanical industries increasingly have sought more refined and efficient devices and techniques in the generation of motion and the effectuation of its control. For example, the mass storage of data for popularly sized computers is carried out by recordation on magnetic disks which are rotatably driven under exacting specifications. The data handling performance of such memory handling systems relies to a considerable degree upon the quality and reliability of the rotational drive components associated with them. Similar requirements for enhanced motion-drive performance are observed to be expanding in the fields such as robotics, machine tools, and the like.
Permanent magnet (P.M.) direct current (d.c.) motors generally have been elected by designers as the more appropriate device for refined motion generation or drive. Clasically, the P.M. d.c. motor is a three-phase device having a stator functioning to mount two or more permanent magnet poles which perform in conjunction with three or more rotor mounted field windings. These windings are positioned over the inward portions of rotor pole structures typically formed of laminated steel sheets. The ends or tip portions of the rotor poles conventionally are flared or curved somewhat broadly to improve their magnetic interaction with the stator magnet. Field windings are intercoupled with either a delta or a Y circuit configuration and by exciting them in a particular sequence, an electromagnetic field, in effect, is caused to move from one flared pole tip to the next to achieve an interaction with the permanent magnets and evolve rotational motion. This interaction occurs in almost all designs through an air gap which is disposed "radially" to the motor shaft between the stator and rotor. The gap for a "radial" configuration is generally in parallel relationship with the axis of the rotor while flux transfer occurs across the gap radially. The interaction between the permanent magnet field and field of the excited windings being one wherein force vectors are developed in consequence of an association of the exciting field with the field or flux of the magnet. Clasically, the switching providing select excitation of the field windings is provided by a commutator rotating with the rotor and associated with brushes representing a make and break mechanical switching device functioning to move the field along the pole tips.
With the advent of more sophisticated electronic systems such as the disk memory assembly of computers, the classic P.M. d.c. motor has been found to be deficient in many aspects. For example, the make and break commutation of historic designs is electrically noisy and somewhat unreliable, conditions which are unacceptable for such applications. Such motors are relatively large, and this aspect contributes to undesired design requirements for bulk, the designer losing much of the desired flexibility for innovation in applications requiring motor drive. Further, the manufacturer of such motors must cope with a somewhat complex nature of typical rotor pole structures carrying field windings. For example, the production of the windings upon individual poles involves a procedure wherein wire is maneuvered beneath the flared tips of a fully assembled rotor structure.
To address the performance limitations of electrical noise caused by brush-type motors, brushless P.M. d.c. motors have been developed wherein field commutation otherwise carried out mechanically has been replaced with an electronic circuit. These motors generally provide a higher quality performance including much quieter electrical performance. Typically, the permanent magnet components of such quieter electrical systems move as opposed to the field windings of the motor and a radial gap architecture is retained from earlier designs. As in these earlier designs, the windings of the brushless motors are provided beneath flared pole tips on the inside of the stator surface and unfortunately retain the noted difficulties of assembly.
Where d.c. motors are configured having steel core poles and associated field windings performing in conjunction with permanent magnets, there occurs a somewhat inherent development of detent torque. At rest, or in a static state, the steel poles of a typical rotor will assume an orientation with respect to associated permanent magnets which develop flux paths of highest density and the least reluctance. Thus, were one to hand rotate the rotor of an unenergized motor of such design, these positions of rest or detent positions can be felt or tactilly detected as well as the magnetic field induced retardation and acceleration developed in the vicinity of the detent positions. During an ensuing excitation state of the motor windings creating rotational drive, this detent torque will be additively and subtractively superimposed upon the operational characteristic of the motor output to distort the energized torque curve, increase ripple torque, reduce the minimum torque available for starting and, possibly develop instantaneous speed variations (ISV) which are generally uncorrectable, for example, by electronics. ISV characteristics also can be generated from mechanical unbalance phenomena in the rotor of the motor itself or the bearings thereof if they are a part of the rotated mass. Generally, detent torque contributions to ISV and other phenomena are observable in the operational characteristic or torque curve of motors, for example being manifested as a form of ripple torque. In the past, the dynamic output of the motors has been smoothed through resort to rotational masses such as flywheels and the like. However, for great numbers of modern applications, design restraints preclude such correction and motors exhibiting large ISV characteristics are found to be unacceptable. As a consequence, spindle motors for disk drives of computer systems, for example, have been configured as vector cross products or B cross I devices, sometimes known as voice coil motors, which do not employ steel pole structures.
Petersen, in U.S. Pat. No. 4,745,345 entitled "D.C. Motor with Axially Disposed Working Flux Gap" issued May 17, 1988, describes a P.M. d.c. motor of a brushless variety employing a rotor-stator pole architecture wherein the working flux gap is disposed "axially" (perpendicularly to the motor axis) and wherein the transfer of flux is parallel with the axis of rotation of the motor. This "axial" architecture further employs the use of field windings which are simply structured being supported from stator pole core members which, in turn, are mounted upon a magnetically permeable base. The windings positioned over the stator pole core members advantageously may be developed upon simple bobbins insertable over the upstanding pole core members. Such axial type motors have exhibited excellent dynamic performance and efficiency and, ideally, may be designed to assume very small and desirably variable configurations. Very often, the space within assemblages made available for retaining the motor function are quite restricted and irregular in general shape. Thus motor design flexibility for such applications represents a subject of increasing interest in industry.
Detent torque characteristics which otherwise might occur with such motor designs are accommodated for by adjusting the geometric design of the permanent magnets within the rotor structure as well as, for example, by developing a skew orientation of the stator core poles. The latter skewing approach, however, necessarily is avoided where the noted design requirements for miniaturization are encountered. Because of the static permanent magnet induced axial forces necessarily present with most such motor structures, accommodation also may be necessary for such forces. These static axial forces may be of such significance as to require the use of "low friction" types of bearings, for example, ball bearings to minimize the d.c. axial force friction effects and permit sufficient starting torques. This requirement also generally imposes the penalty of a larger and more costly bearing structure than might otherwise be required. The designer may also be called upon to address any time varying force term generated in consequence of commutation of the motors. Without such accommodation, for example, noise may be generated which for some applications will be found undesirable.
Another aspect of the design for such P.M. d.c. motors which occurs in conjunction with computer disk drive applications resides in the desirability of effecting a seal of the motor components from the disk-head environment. This requirement follows from the disk environment wherein, for example, a hard disk slider generally floats 20 millionths of an inch or less above the disk surface which spins, for example, at a rate of about 150 mph. The dynamics of such a structure have been likened to a condition wherein an airliner is flown six inches off the ground. A slight obstruction such as a smoke particle can interrupt disk operation. Thus, it is desirable that the drive motors themselves contain some form of a seal which does not detract from their operational efficiency.